1. Field of the Invention
The present invention relates to electrically operated spool valves that control flow of a fluid, and more particularly to electrical actuators, such as solenoids, for operating those valves.
2. Description of the Related Art
A wide variety of machines have moveable members that are operated by an hydraulic actuator. For example an internal combustion engine has a camshaft which is mechanically coupled to rotate with the crankshaft and which opens and closed cylinder intake and exhaust valves. Traditionally the camshaft timing was fixed at a setting that produced the best operation at all engine operating speeds. However, it has been recognized that engine performance can be improved if the valve timing varies as a function of engine speed, engine load, and other factors. Thus a hydraulic actuator is being used on some engines to vary the coupling relationship of the camshaft to the crankshaft and a solenoid operated valve is employed to control the application of pressurized fluid to operate the hydraulic actuator.
U.S. Pat. No. 7,007,925 discloses one type of solenoid operated valve that has been used to vary the timing of an internal combustion engine. A unique feature of this valve is that the armature assembly of the solenoid has a ball bearing which reduces resistance to movement of the armature. The armature assembly included the metal armature from which a push pin projected to engage and move a flow control spool of the valve. The ball bearing comprised a cylindrical cage that held a plurality of balls inserted through openings in one end of the cage. The cage was slid over the push pin until the openings were against the armature to retain the balls and then the cage was secured to the push member by a push-on nut. When the solenoid is assembled the armature assembly moved within a bore and the balls rolled along the surface of the bore. Although the ball bearing worked very well, it added complexity to the valve assembly process.
Therefore, it is desirable to refine the design of this type of solenoid operated valve to facilitate manufacturing.